Optical fibre is a self-evident choice as a transmission medium for a core network, since trunk connections usually require high transmission capacity, the transmission distances used are long, and ready routes are often available for cables. Also for subscriber connections (line between a local exchange and a subscriber) the situation is rapidly changing, since various services implemented with multimedia, requiring a high transmission rate, will be commonplace from the point of an individual consumer as well.
However, no significant savings in the costs for constructing a future network offering broadband services can be foreseen, since the costs mainly arise from cable installation costs. However, it is desired to construct optical fibre also in the subscriber network as much as possible, since it is clearly seen that there will be a demand for it in the future. The costs of renewing subscriber networks are very high, however, and in terms of time, decades are actually involved in this context. High costs are indeed the principal impediment to the spreading of fibre to the subscriber network.
On account of the above reasons, the possibilities of utilizing conventional subscriber line (metal pair cable) for fast data transmission, i.e. for rates clearly above the rate of an ISDN basic access (144 kbit/s), have been mapped out more effectively than before. The present ADSL (Asymmetrical Digital Subscriber Line) and HDSL (High bit rate Digital Subscriber Line) techniques do offer new possibilities for transfer of fast data and video along the pair cable of a telephone network to subscriber terminals.
An ADSL transmission link is asymmetric in the sense that the transmission rate from network to subscriber is much higher than from subscriber to network. ADSL technology is mainly intended for various on-demand services. In practice, the rate of an ADSL transmission link from network to subscriber is in the order 2-6 Mbit/s and from subscriber to network in the order 32-640 kbit/s (mere control channel). (The data rate of an ADSL line is always n×32 kbit/s, where n is an integer.)
The HDSL transmission technique relates to the transfer of a 2 Mbit/s-level digital signal in a metal pair cable. HDSL transmission is symmetric, that is, the transmission rate is the same in both directions.
Since the above solutions only afford rates in the order 1-6 Mbit/s, a technique enabling ATM-level rates (10-55 Mbit/s) has also been sought for the pair cable of a subscriber line. The international standardization body ETSI (European Telecommunications Standards Institute) is working out a specification on VDSL (Very high data rate Digital Subscriber Line) equipment enabling such rates. By VDSL technology, both symmetric and asymmetric links can be implemented.
The above technologies, by which fast data is transferred through a pair cable, are called by the common name xDSL. Thus, even though it is not yet possible to offer broadband services to end users by utilizing optical fibre, by means of these techniques the present telephone operators are capable of offering said services through existing subscriber lines.
Since ADSL seems at the moment to be the most promising technique for implementing broadband services, it will be used as an example of the access technique by means of which the services are offered.
The ADSL Forum has specified a generic network model for xDSL links; this is illustrated in FIG. 1. The device that connects to a subscriber line at the subscriber end is called ATU-R (ADSL Transmission Unit—Remote), and the device that connects to a subscriber line at the network end (e.g. at a local exchange) is called ATU-C (ADSL Transmission Unit—Central). These devices are also called ADSL modems (or ADSL transceivers), and they define between them an ADSL link. The high-speed data on the ADSL link is connected to the subscriber line in such a way that the subscriber can still use the old narrowband POTS/ISDN services, but the subscriber additionally has a high-speed data connection available. In principle, there are two ways to multiplex POTS and ADSL signals or ISDN and ADSL signals onto the same subscriber line: time division multiplexing or frequency division multiplexing. The present invention employs frequency division multiplexing, in which narrowband and broadband services are separated from one another by a splitter or cross-over carrying out the frequency division of ADSL signals and narrowband signals. The splitter can be a POTS/ADSL splitter PS or an ISDN/ADSL splitter IS.
The terminals TE at the end user can be of many different types, such as terminals TE1 of a cable TV network, personal computers TE2 or even ISDN phones TE3 if time division multiplexing is used. A service module SMi (i=1 . . . 3) is provided for each terminal, carrying out the functions relating to terminal adaptation. Such service modules can in practice include Set Top Boxes, PC interfaces or LAN routers, for example. A premises distribution network PDN, located at the premises of the subscriber, connects the ATU-R to the service modules.
At the network end of the ADSL link, an access node AN constitutes a concentration point for narrowband and broadband data, at which point the traffic arriving from different service systems through different networks is concentrated. The access node is located at the exchange of a telephone network, for example.
In FIG. 1, reference A denotes the part constituted by a private network, B the part constituted by a public network, and C a network located at the premises of a subscriber (the telephones are naturally located at the subscriber).
The generic network model relating to xDSL links was set forth above in order to describe the overall environment of the invention. Since the invention relates to the part constituted by the actual ADSL link, which is located either between the local exchange and the subscriber or between a street cabinet and the subscriber, only this part located between the ADSL modems will be described in closer detail hereinbelow.
As stated previously, POTS (Plain Old Telephone Service) and ADSL services can be frequency multiplexed onto the same pair cable by means of a splitter. FIG. 2 illustrates a subscriber line divided between POTS and ADSL services, denoted with reference SL. In practice, the splitter (PS1 or PS2) comprises two filter units: a low-pass filter unit LPF prevents the access of signals of the ADSL band (25 kHz . . . 1.1 MHz) to the POTS interface I1, and a high-pass filter unit HPF prevents the access of signals of the POTS band (0 Hz . . . 4 kHz) to the ADSL interface I2. Thus, the frequency division of the link is of the kind shown in FIG. 3: signals relating to POTS or ISDN services are transferred at low frequencies, and ADSL signals are transferred at higher frequencies. The splitter has a line port (P) connected to the subscriber line. The low-pass filter unit is connected between the line port and the POTS interface I1, and the high-pass filter unit HPF is connected between the line port and the ADSL interface I2.
Teleoperators determine the viability of filters by means of a reference impedance, which is defined so as to correspond to the actual impedance of the subscriber link as well as possible. FIG. 4 depicts a typical reference impedance Zref used by operators, comprising a resistance (R11) followed by a parallel connection of a resistance (R12) and a capacitor (C11). Some operators define the reference impedance as real (R11=C11=0), but in a generic case, however, the reference impedance is complex. The filter must provide a sufficiently good impedance match to the reference impedance in the voice band. A perfect impedance match is achieved when the output impedance of the generator concurs with the load impedance. The operators estimate the viability of filter units by feeding to a load impedance, being equal to the reference impedance, a signal from a generator whose output impedance also equals the reference impedance. The load impedance is looked at through the filter. In such a case, the effective load impedance deviates from the reference impedance, since the filter unit can never be entirely transparent. There is no international standard relating to the impedance match, but each operator has his own quantitative measure in determining what is a sufficiently good impedance match.
The impedance match of the splitter must be as good as possible in both directions, that is, too much reflection is not allowed in either direction. In practice, such a filter unit can be implemented as a passive LC network (i.e., as a circuitry comprising windings and capacitors). In the case of a high-pass filter, LC implementation seems to be the only feasible alternative. In the case of a low-pass filter, however, this implementation is attended by significant problems, which will be described in the following.
A passive low-pass filter loads both the subscriber line and the POTS interface by its input impedance. This impedance should equal the impedance of the subscriber line and the POTS interface as well as possible before the installation of the splitter, since in that case the splitter does not impair the matching of the POTS interface to the subscriber line. However, the impedance of a passive filter cannot be designed independently of the other parameters, but the transfer function striven for and the load impedance set the boundary conditions for the impedance to be realized.
Furthermore, sufficient isolation must be provided between the POTS and ADSL services. It has been found in practice that an insertion loss of at least 40 dB is needed in the low-pass filter to ensure that the services appear to the subscriber as fully isolated from one another. Such attenuation also meets the international requirements set on the maximum signal level outside the voice band, measured at a POTS interface. In the case of most operators, however, it is not possible to achieve a sufficiently good impedance match in such a situation. The resultant mismatch diminishes the trans-hybrid loss at the exchange end and sidetone masking at the subscriber end, thus deteriorating service quality.
Hence, the characteristic impedance Z′0 that is visible when the subscriber line is seen through an LC filter is in practice at least somewhat dissimilar to the characteristic impedance Z0 of the plain subscriber line. However, an attempt must be made to carry out the filtering so that Z′0≈Z0. There are two principal alternatives for realizing the impedance matching. First, the actual filter can be implemented in such a way as to exhibit a sufficiently correct input impedance. Another alternative is to implement the filter in such a way that its input impedance clearly deviates from the ideal value, but the impedance matching is corrected by a discrete correction block. Such a correction block is termed a Generalized Immittance Converter (GIC). The alternative to be selected for the implementation is mainly dependent on the standard specifications set by the teleoperators. Some teleoperators stipulate in their specifications that the impedance matching be effected by means of a discrete GIC block.
A solution based on the use of a GIC block is disclosed in European patent publication EP-0742972-B1. This publication discloses a POTS splitter in which a passive filter (LC network) is used as a low-pass filter, but in addition to this, a two-way impedance correction is made by using two GIC blocks. More specifically, in the implementation disclosed in said publication the LC and GIC blocks have been used as shown in FIG. 5, i.e. in such a way that the LC network 52 serving as a low-pass filter is placed between two GIC blocks 51. Hence, in this publication the starting-point is the idea that to make the impedance matching in each direction sufficiently good, the low-pass filter must be implemented as reciprocal, i.e., mirrored.
However, in practice a GIC block is a costly and sizeable circuit element, and hence the inclusion of two such substantially similar circuit elements will render the splitter expensive and large and result in a cumbersome practical implementation.
Furthermore, a GIC block has been found to be associated with at least the following drawbacks:                In the preferred practical implementation, the input circuit of the GIC block will connect to line capacitively and the output circuit inductively. In such a case, a limited amount of transfer resistance is associated with the GIC block, which will result in an increase in the effective length of the POTS line in offhook state.        A limited amount of shunt impedance is also associated with the GIC block, which will load the line in offhook state in which the impedance is nominally infinite.        The GIC block is active, i.e. consumes power.        